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MICRORESISTIVITY BASICS
Microresistivity logs are miniature
versions of the ES Log, laterolog, and later offshoots of this
technology. The microlog arrived in 1948 and the microlaterolog in
1952; both were invented by Henry Doll of Schlumberger. Additional
tools were developed to improve performance: the proximity log and
microspherically focused log for example. All had electrodes
implanted on a small pad pushed against the face of the wellbore.
They are designed to measure only an inch or two vertically, with a
similar depth of investigation. They can still be run today, but few
are, which is a shame as they offer a cheap thin bed log. The
resistivity or acoustic image logs are better for
the purpose but more expensive to run.

All microresistivity logs require a conductive mud in
open hole and do not work in cased hole.

All microresistivity logs are based on the
application of Ohm's Law,
adapted for the peculiar geometry of the asymmetry of an electrode
arrangement pressed to one side of a well bore. .

The Microlog tool is a shallow resistivity
device mounted in an oil-filled rubber pad which is pressed to
the borehole wall during logging by hydraulic pressure electrically
controlled from the surface. The pad can be considered as a miniature
electrical survey.

The illustration below shows the Microlog electrode arrangement.
Three small button electrodes (A, M1 and M2), spaced one inch
apart, are embedded in the center of the insulated pad. A remote
electrode, usually near the pad, is also used serving as the
current return electrode B and voltage reference electrode
N.

Current electrode A is maintained
at constant current intensity. The potential difference between
electrodes M1 and M2 is used to derive a resistivity curve
which is called the micro-inverse and is usually designated
R1x1, or simply R1. The electrode arrangement for R1 is equivalent
to a lateral-type resistivity tool having a depth of investigation
of 1.5 inches.

The potential difference between electrode
M2 and the reference or remote electrode is used to derive
a second resistivity curve called the micro-normal, usually
designated R2. The electrode arrangement for R2 is equivalent
to a normal resistivity tool and its 2 inch spacing gives it
a depth of investigation of two to four inches.

The Microlaterolog is a focused, pad-mounted
shallow resistivity tool developed to overcome limitations
of the Microlog in high resistivity formations and in salt
mud situations. The tool design is similar to the Microlog
with the exception of the electrode arrangement (see
illustration above). The electrode arrangement consists of a current electrode
in the center surrounded by focusing electrodes embedded in
an oil-filled rubber pad. A remote current return electrode
is located near the pad. This electrode arrangement is similar
to the seven electrode laterolog on a miniature scale.

A voltage of constant intensity is
applied to the center electrode while a controlled supply of
current is applied to the focusing electrodes. The potential
difference between the center electrode and the focusing or
guard electrodes is maintained at zero by automatic controls.
This has the effect of focusing the current into a narrow beam
perpendicular to the pad and into the formation. The current
beam maintains a uniform shape through the mud cake, spreading
out as distance from the pad increases.

The potential difference between the
center electrode and the remote electrode, in combination with
a calibration constant, is a measure of the apparent resistivity
of a small volume of the formation near the borehole. The depth
of investigation is about three inches from the tool pad. For
mud cake less than 3/8 inch thick, the effect of mud cake on
tool response is small and can generally be ignored. With flushed
zone thickness of two to three inches. The tool reads flushed
zone resistivity (Rxo) directly.

In wells drilled with low resistivity
(salt) muds, mud cake resistivity is usually quite low compared
to flushed zone resistivity. Under these conditions the mud
cake effect is still small for mud cake thickness greater than
3/8 inch. For fresher muds and higher Rmc/Rxo ratios, the 3/8
inch limitation applies.

The Proximity log is a focused, pad-mounted
tool and is a further development of the Microlaterolog to
minimize mud cake effects.

The tool design is very similar except
for a modified guard electrode arrangement. A second ring of
guard electrodes, in addition to those used in the Microlaterolog
arrangement, is included. The beam electrode and the guard
electrodes also have larger cross-section areas. This configuration is referred to as a shielded guard
device.

A voltage of constant intensity is applied to the beam
electrode. A controlled supply of current is applied to the
guard electrode to maintain zero potential between the shield
and the beam electrode. The additional focusing shield constricts
the current beam from the center electrode even more than with
the Microlaterolog tool. A greater thickness of mud cake is
thus penetrated with little change in the shape of the current
beam from the center electrode.

Measurement of the potential difference between the
center electrode and the remote return electrode in combination
with a calibration constant gives the resistivity of a small
volume of the formation. The improved focusing gives the Proximity
Log a greater depth of investigation and most of the tool response
is received from a distance of six to ten inches from the pad.
Field tests indicate that where moderate to deep invasion exists
and sufficient flushing has occurred, reliable values for the
flushed zone resistivity (Rxo) may be obtained.

The Micro Spherically Focused Log
(MSFL) has superceded the Proximity log. It has the general
electrode arrangement of the SFL described earlier, placed
on a pad in miniature form. The MSFL is the current tool of
choice for flushed zone Rxo measurement.

ES log (left) with Microlog (right). Shaded areas
show “positive separation” where 1” inverse
(solid line) is less than 2” normal (dashed line).
This is an indication of porous, permeable reservoir rock.
High resistivity is tight; low resistivity with no significant
separation is shale. Micrologs are still run routinely
today and are still a great reservoir finder.